Method for forming thin film with a gas cluster ion beam

ABSTRACT

A method of forming a thin film on the surface of a substrate such as silicon, in which a gas cluster (which is a massive atomic or molecular group of a reactive substance taking the gaseous form at room temperature under atmospheric pressure) is formed and then ionized, and the cluster ions are then irradiated onto a substrate surface under an acceleration voltage to cause a reaction. 
     It is possible to form a high quality ultra-thin film having a very smooth interface, without causing any damage to the substrate, even at room temperature.

This is a continuation of Ser. No. 09/916,304, filed Jul. 30, 2001, nowabandoned, which is a continuation of Ser. No. 08/946,663, flied Oct. 7,1997, now abandoned, which is a continuation of Ser. No. 08/757,088,filed Nov. 26, 1996, now abandoned, which is a continuation of Ser. No.08/523,922, filed September 6, 1995, now abandoned.

FIELD OF THE INVENTION

The present invention relates to a method for forming a thin film on thesurface of a substrate with a gas cluster ion beam. More particularly,the present invention relates to a method for forming a thin film by agas cluster ion beam which is a group of reactive substances gaseous atthe room temperature and under atmospheric pressure. The method isuseful for the manufacture of a semiconductor or other electronicdevices and for surface reforming of a functional material.

PRIOR ART AND PROBLEMS

A method for forming a thin film by irradiating monatomic ormonomolecular ions onto a substrate surface has conventionally been usedin practice. This method utilizes a high input energy of several keVbecause low-energy ion irradiation cannot give an adequate beam becauseof the space charge effect between ions.

In this conventional method, however, the use of ions having a largeinput energy makes it difficult to avoid damage to the substratesurface, and thus deterioration of semiconductor devices has been amajor problem.

As a method for forming a thin film by causing reaction between areactive substance, which is gaseous at the room temperature, and asemiconductor substrate, a thermal reaction method has been developedfor forming an oxide or nitride film by heating the substrate to a hightemperature in an atmosphere of the reactive substance. The thin filmformed by this method has excellent interface and insulation properties,and the method has been industrialized for forming an insulating film ora capacitor insulating film for silicon semiconductor devices.

This method requires a low temperature to reduce the diffusion ofimpurities, which is desirable for integrated circuit devices, and thechemical vapor deposition (CVD) method is adopted for this purpose.

In this case, however, it is necessary to heat the substrate surface toa temperature of at least 400° C., but this results in defects such as alow density of the resultant film as compared with that produced by thethermal reaction method and the presence of many unsaturated bonds onthe interface between the substrate and the thin film.

The plasma CVD method is known as a method for forming a thin film at alow temperature. This method however involves such defects in that itintroduces a large amount of mixed impurities in the substrate, causesdamage to the substrate surface by the ions, and results in difficultiescontrolling the film thickness of an ultra-thin film. Furthermore, it isnot applicable to a transistor gate insulating film or a capacitorinsulating film requiring high quality.

In the conventional techniques, as described above, the quality of theresultant thin film deteriorates as the process temperature becomeslower, and it is difficult to obtain an ultra-thin film useful for ahyperfine semiconductor circuit device such as ULSI.

There has consequently been a strong demand for a new method whichpermits formation of a high-quality thin film at a lower temperature,particularly at room temperature, without the need to heat the substrateand without damaging the substrate surface, as a fundamental technologyfor use in advanced electronics such as ULSI.

The present invention was developed in view of the circumstances asdescribed above, and has an object to provide a method for forming ahigh-quality ultra-thin film having a smooth interface with thesubstrate free from a damage at the room temperature, which solves thedrawbacks in the conventional methods for forming a thin film.

SUMMARY OF THE INVENTION

The present invention provides a method for forming a thin film on thesurface of substrate with a gas cluster ion beam, which comprises thestep of irradiating the surface of a substrate with ions of a gascluster (which is a massive group of atoms or molecules of a reactivesubstance in gaseous form at room temperature and under atmosphericpressure) to cause a reaction with a substance of the substrate surface,thereby forming a thin film on the substrate surface.

More specifically, the invention comprises the steps of using a reactivesubstance selected from the group consisting of oxides, nitrides,carbides, mixtures thereof, and mixtures thereof with an inert gas andcausing a reaction between a gas cluster ion beam of this substance anda substance of the substrate surface, thereby forming a thin film on thesubstrate surface.

The invention provides also a method for forming a thin film based on agas cluster ion beam, which comprises the steps of irradiating thesurface of the substrate with a gas cluster ion beam to form a thin filmand at the same time to planarize the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows transmission type electron microscopic photomicrographs ofa section of a silicon substrate by irradiating CO₂ monomers and CO₂cluster ions onto the substrate surface thereof;

FIG. 2 shows an infrared absorption spectrum of a thin film formed byirradiating CO₂ monomer ions and CO₂ cluster ions onto a siliconsubstrate surface;

FIG. 3 shows a chart illustrating the relationship between the dose rateof CO₂ ions to be irradiated and the concentration of impurities in thesubstrate surface;

FIG. 4 shows a diagram illustrating the generation of clusters ofvarious gases by means of a nozzle having cooling means;

FIG. 5 shows a graph illustrating the relationship between the supplypressure and the cluster beam intensity at various nozzle intensitiesfor oxygen gas;

FIG. 6 shows a graph illustrating the relationship between the supplypressure and the cluster beam intensity at various nozzle intensitiesfor nitrogen gas;

FIG. 7 shows a graph illustrating the relationship between the supplypressure and the cluster beam intensity at various nozzle intensitiesfor argon gas for reference; and

FIG. 8 shows an infrared absorption spectrum of an oxide film formed byirradiating oxygen (O₂) gas cluster onto a silicon substrate surface.

DETAILED DESCRIPTION OF THE INVENTION

The method of the invention comprises generating a gas cluster (which isa massive group of atoms or molecules of a reactive substance which is agas at room temperature under atmospheric pressure), irradiatingelectrons onto this gas cluster to ionize same, and irradiating the thusgenerated gas cluster ions onto a substrate surface by selecting a beamof a particular size as required.

Irradiation is accomplished by accelerating the gas cluster ions underan acceleration voltage. Because a cluster usually consists of a groupof several hundred atoms or molecules, each atom or molecule isirradiated as an extra-low temperature ion beam with UP to several tensof eV and even under an acceleration voltage of 10 kV. It is thereforepossible to form a high-quality ultra-thin film with a very low degreeof damage at room temperature. A thin film with the slightest content ofimpurities can be produced as the impurities are removed from thesubstrate surface by the effect of the gas cluster ions.

Another effective manner of irradiation is to appropriately select thenumber of component molecules in response to the substrate, the kind ofsubstance of the substrate surface and the desired thin filmcomposition.

The cluster itself can be generated by ejecting a pressurized gasthrough an expansion type nozzle into a vacuum unit, as already proposedby the present inventors. The thus generated cluster can be ionized byirradiating electrons.

In the present invention, the gas cluster ions are irradiated onto asubstrate surface to planarize the surface and at the same time, theirradiated ions are caused to react with the substance of the substratesurface, thereby permitting formation of a thin film on the substratesurface.

The gaseous reactive substances taking a gaseous form at roomtemperature under the atmospheric pressure include, for example, oxygen,oxides such as CO₂, CO, N₂O, NO_(x), and C_(x)H_(y)O_(z), carbides,nitrides such as N₂ and NH_(x), sulfides, halides, hydrides such asAsH_(x) and SiH_(x), organometallic compounds such as metal carbonyls.

In the present invention, for example, the reactive substance ispreferably oxygen or a carbon oxide, or a mixture thereof, or a mixturethereof with an inert gas substance, and the thin film formed can be anoxide film.

In the case of any of the gases including oxygen (O₂) and nitrogen (N₂),the present invention specifically proposes a method for generating gascluster ions, which comprises the step of generating a gas cluster bymeans of a nozzle cooled by a coolant, and ionizing the resultantcluster.

The invention will be described in further detail by the following ofexamples.

EXAMPLE 1

FIG. 1 is transmission type electron microscopic photomicrographs of asilicon substrate section when (a) CO₂ monomolecular ions wereirradiated onto an silicon substrate at a dose rate of 1×10¹⁶ ions/cm²,and (b) CO₂ cluster ions having a number of component molecules (clustersize) of at least 500 were irradiated.

In FIG. 1 (a), CO₂ monomolecular ions were irradiated under anacceleration voltage of 10 kV onto a silicon (001) substrate. Anamorphous silicon layer of a thickness of 19 nm was formed on thesubstrate surface, and irregularities occurred on the interface betweenthe amorphous layer and the substrate. The amorphous silicon layer is adamaged layer formed when CO₂ ions ejected onto the substrate surfacehit atoms of the substrate. These irregularities of the interface andthe damaged layer, causing deterioration of semiconductor deviceproperties, must be converted to monocrystalline state. It is howeververy difficult to achieve complete conversion even through heattreatment at a temperature of at least 800° C.

In the case of FIG. 1(b), on the other hand, CO₂ cluster ions having anumber of component atoms (cluster size) of at least 500 wereaccelerated under conditions including a dose rate of 1×10¹⁶ ions/cm²and an acceleration voltage of 10 kV, and then, only clusters of anumber of component molecules of at least 500 were irradiated onto asilicon (001) substrate at room temperature by the retarding-fieldmethod. A silicon oxide film having a thickness of 10 nm was formed onthe substrate surface, and no damaged layer was observed between thesilicon oxide film and the silicon substrate, and the interface inbetween was very smooth.

The above-mentioned findings are confirmed also from FIG. 2. FIG. 2 isan infrared absorption spectrum of a thin film formed by irradiating CO₂monomer ions and cluster ions under conditions including an accelerationvoltage of 10 kV and a dose rate of 2×10¹⁶ ions/cm² onto a siliconsubstrate surface. While a silicon oxide film only of the order ofspontaneous oxide film can be formed by irradiation with CO₂ monomerions, irradiation with cluster ion's permits formation of an oxide filmof about 70 A.

EXAMPLE 2

FIG. 3 illustrates changes in the concentration of Ni impurities asmeasured with various dose rates of CO₂ ions irradiated onto a siliconsubstrate, ranging from 0 to 2×10¹⁵ ions/cm². In this case, CO₂ clusterions having a number of component molecules of at least 250 and CO₂monomer ions were irradiated under an acceleration voltage of 10 kV ontothe silicon substrates to which Ni had previously been forcedlydeposited to give of 6×10¹² atoms/cm², and changes in the concentrationof Ni impurities was measured before and after irradiation throughmeasurement by the total reflection X-ray flourescence analysis method.While the concentration of Ni impurities does not depend upon the extentof dose rate in the irradiation of monomer ions, the concentration of Niimpurities decreases along with the increase in the dose rate in theirradiation of cluster ions.

As is clear from these results, in the case of irradiation of clusterions, it is possible to reduce the concentration of impurities byincreasing the dose rate of irradiated ions. The higherimpurities-removing effect in the irradiation of cluster ions ascompared to the irradiation of monomer (monomolecular) ions isattributable to the fact that, while embedding of impurities adhering tothe surface into the interior of the substrate occurs in the irradiationof monomer ions, sputtering from the irradiation onto the substratepreferentially eliminates impurities on the substrate surface in theirradiation of cluster ions.

As described above, the method of the present invention, permittingformation of a clean surface free from defects on the substrate surface,makes it possible to manufacture a high-quality thin film with theslightest content of impurities, useful for a semiconductor circuitdevice.

EXAMPLE 3

Table 1 shows, for an SiO₂ film formed on a polycrystalline silicon filmirradiated with CO₂ cluster ions under conditions including anacceleration voltage of 10 kV and a dose rate of 5×10¹⁵ ions/cm², thefilm thickness and the average surface roughness before and after atreatment with fluoric acid solution. The treatment with fluoric acidsolution was applied until complete elimination of the SiO₂ film. Forcomparison purposes, values obtained with a polycrystalline silicon filmnot irradiated with cluster ions are also shown.

TABLE 1 Average surface roughness (unit: Å) Before After Thickness offluoric fluoric SiO₂ acid acid Kind of sample thin film (nm) treatmenttreatment Sample not irradiated — 37 37 Sample irradiated with 8 7 9cluster ions of size of 250 Sample irradiated with 6 18 20 cluster ionsof size of 500

The sample not irradiated with cluster ions has an average surfaceroughness of 37 A, and this value does not vary with the fluoric acidtreatment. Irradiation of cluster ions of a size of 250 and 500 reducesthe average surface roughness of the polycrystalline silicon film to 7 Åand 18 Å, respectively. At the same time, an SiO₂ thin film having athickness of from 8 to 6 nm is formed on the surface of thepolycrystalline silicon film, thus forming an SiO₂ thin filmsimultaneously with planarization. A satisfactory surface planarity iskept with almost no change by the fluoric acid treatment. Thus, theirradiation of CO₂ cluster ions permits formation of a silicon oxidefilm which has a planar surface, with a smooth interface between saidfilm and the silicon substrate, and has a uniform thickness.

EXAMPLE 4

Conditions for generating clusters of various gases were evaluated. Asshown in FIG. 4, cooled dry nitrogen gas was passed through a piping (2)attached to a nozzle (1) section which was then cooled. This permittedionization of a cluster of gases which had not been possible at roomtemperature.

FIG. 5 is a graph illustrating the relationship between the intensity ofan oxygen (O₂) gas cluster beam and the supply pressure at variousnozzle temperatures. FIG. 6 is a graph illustrating such relationshipfor nitrogen (N₂) gas. FIG. 7 illustrates the relationship for Ar gasfor reference.

These results suggest the possibility of determining the degree ofeasiness of generation of the cluster of a gas applicable in the presentinvention by means of the following formula:

Easiness of generation of cluster

Ψ=P ₀ ·d ₀·(T _(b) /T _(N))γ/(γ−1)

where, P₀: gas supply pressure,

d₀: inside diameter of nozzle throat,

T_(b): boiling point of gas,

T_(N): ejecting temperature of gas,

γ: ratio of specific heats of gas (specific heat at constantpressure/isometric specific heat).

Larger P₀ or T_(b) leads to easier generation of clusters.

Smaller T_(N) or γ corresponds to easier generation of clusters.

In all the cases described above, various gases were effectivelyutilized for the formation of thin films.

EXAMPLE 5

FIG. 8 is an infrared absorption spectrum of a thin film formed byirradiating O₂ cluster ions having a number of component molecules(cluster size) of at least 250 onto an Si substrate surface at roomtemperature under an acceleration voltage of 4 kV and at a dose rate of1×10¹⁵ ions/cm². It is clear from FIG. 8 that an oxide film of about 40A was formed with a lower acceleration voltage and a lower dose ratethan the conditions for irradiation of CO₂ cluster ions.

EXAMPLE 6

Table 2 shows the results of investigation by the photoelectronspectroscopy method of an oxide film formed by irradiating CO₂ clusterions and O₂ cluster ions under the same irradiating conditions includingan acceleration voltage of 4 kV, a cluster size of at least 250, and adose rate of 1×10¹⁵ ions/cm². The results of investigation on a clean Sisubstrate not irradiated are also shown for reference. By using O₂cluster ions, it is possible to form a thicker oxide film as comparedwith the case of CO₂ cluster ions. It is also clear that the relativeintensity of emission spectrum from Cls core level of the oxide filmformed by the irradiation of O₂ clusters is of the same order as thatfor a substrate not irradiated and the carbon content is almost nil,even with the largest film thickness. When the presence of residualcarbon in the film is not desired, it is suggested to use O₂ clusterions to obtain a thicker oxide film.

TABLE 2 Raw material gas Oxide film of irradiated thickness Relativeintensity cluster ions (unit: A) of Cls signal O₂ 40 31 CO₂ 18 44Substrate not  8 29 irradiated

According to the present invention, as described above in detail, it ispossible to form a high-quality reactive ultra-thin film, with no damageto the substrate, having a very smooth interface, even at the roomtemperature, by irradiating cluster ions of a reactive substance ontothe substrate surface to cause a reaction.

What is claimed is:
 1. A method for forming a thin film on a surface ofa substrate with a gas cluster ion beam, which comprises irradiating thesurface of a substrate with ions of a gas cluster of a gas comprisingoxygen (O₂) and optionally an inert gas, to cause a reaction of saidions of the gas cluster with the surface of the substrate to therebyform the thin film on the substrate surface, said gas cluster being acluster of atoms or molecules which are in a gaseous state at roomtemperature and under atmospheric pressure, which when ionized to formsaid gas cluster beam, react with the surface of the substrate.
 2. Themethod as claimed in claim 1, wherein the said gas cluster is formedfrom said oxygen in admixture with an inert gas.
 3. The method asclaimed in claim 1, wherein the gas cluster ions are selected in termsof a number of component molecules, dependent upon the chemicalcomposition of the substrate, to produce the thin film on the substrate.4. The method as claimed in claim 1, wherein impurities present on thesubstrate are removed by the irradiation with the gas cluster ion beam,thereby forming said thin film with a lower content of impurities thanbefore said irradiation.
 5. The method as claimed in claim 1, whereinthe substrate surface is flattened by the irradiation with the gascluster ions, at the same time as the thin film is formed.
 6. The methodas claimed in claim 1, wherein the thin film formed is an oxide.
 7. Themethod as claimed in claim 1, wherein the substrate is silicon.
 8. Themethod as claimed in claim 1, wherein the gas cluster is generated bysupplying a pressurized gas to an expansion type nozzle cooled by acoolant.
 9. A method for forming a thin film on a surface of a substratewith a gas cluster ion beam, said gas cluster comprising oxygen (O₂) andoptionally an inert gas, which includes the steps of: (i) forming saidgas cluster from a substance which, upon being ionized, is reactive withthe surface of the substrate, (ii) ionizing the gas cluster to form saidgas cluster ions, and (iii) irradiating the surface of the substratewith said gas cluster ion beam.